Why did slavery in the Deep South grow so rapidly between 1789 and 1838?

Slavery did not become a force in the northern colonies mainly because of economic reasons. Cold weather and poor soil could not support such a farm economy as was found in the South. As a result, the North came to depend on manufacturing and trade.
People had been growing and using cotton forever, but it was difficult to get a lot of it. It was taxing to pick and difficult to process. The fields where it grew best were hot, wet, pestilential and prone to disease; you couldn’t pay workers enough to work in them.
But gradually, in the early years of the 18th century, shipping and weaving methods became good enough to make cotton profitable enough that a second option became available.
You still couldn’t pay people enough to work in the cotton fields, but you COULD hire guards and pay THEM enough to FORCE other people to do it.
Slavery was a business.